The DSL technology provides fast data transfer over ordinary telephone lines. It is very advantageous to be able to use telephone lines, i.e. an already existing infrastructure, since almost worldwide customer premises already have telephone lines installed which means that installation costs will be minimized both as far as actual installation costs are concerned and as far invested time is concerned. A disadvantage however is that a telephone line is not or was not originally intended for high speed data transfer and the status of copper wires is often unknown since they may be very old, damaged. In addition thereto they are easily affected by disturbances since they are relatively unprotected. The cables may be particularly exposed if they are so called aerials or unshielded cables. Also untwisted in-house wiring or other wiring in-house of inferior quality is generally vulnerable. It is therefore important that the telephone lines can be appropriately supervised in order to enable detection of errors and to avoid disruption of services. Measurements are important for trouble shooting. Examples on DSL technologies are ADSL (Asynchronous Digital Subscriber Line) and VDSL (Very High Speed Digital Subscriber Line), for example ADSL 2, 2+ or VDSL 2. When applying DSL technology in general and ADSL 2/2+ or VDSL 2 in particular, there are certain limitations due to the characteristics of the twisted pair cable or loop carrying the DSL signals, and it generally consists of copper. These characteristics may affect the conditions for reception/ingress of non-wanted noise. Normally the noise conditions are well known and documented for different types of twisted pair loops or pair cables. The most common form of ingress of noise into a typical twisted pair cable is noise originating from a neighbouring loop. This is called crosstalk and introduces static or semi-static colored noise on the loop. The loop may also be exposed to ingress of other types of noise, for example non-stationary types of disturbances, so called impulse noise. The impulse noise may originate from many different types of sources such as in-house dimmers, various electrical devices, internal combustion engines etc. This makes it difficult to characterize such noise since there is a large variety of sources producing the noise which means that the noise may be shaped in many different ways.
Impulse noise on a twisted pair DSL loop causes instability to the transmission of data and may lead to drop of synchronization of the DSL signalling, which in turn may result in so called re-initializations. DSL standards, cf. for example ITU-T recommendation G.992.3. 992.5, specify some typical characteristics of different types of impulse noises. There are methods known to reduce the effects of impulse noise. By applying Forward Error Correction (FEC) coding and spreading errors in time using interleaving of various depths, the DSL signal (generated by a DSL transceiver) can be protected against impulse noise. Specific combinations of parameters of the FEC and interleaver in DSL standards for the purpose of providing protection against impulse noise are called INP (Impulse Noise Protection), which is an abstract parameter that can be set on different levels.
However, when impulse noise protection is activated on a DSL transceiver both the bitrate throughput and the transmission delay will suffer therefrom. In order not to suffer from too severe performance losses, it is important to be able to limit impulse noise protection as much as possible and not use it to a larger extent than absolutely necessary. This is very difficult and it in turn means that is important to be able to characterize and detect impulse noise. It is also important to find out and locate the loops in larger networks that do suffer from impulse noise. Today it can be established if the impulse noise is repetitive, known as Repetitive Impulse Noise, REIN, and also to determine the repetitiveness, i.e. the frequency. However, the frequency is only one of the parameters characterizing impulse noise which also is characterized by other parameters, which tend to be even more important. The repetitiveness of the impulse noise can e.g. be detected using a measurement method described in “Management of a DSL copper network using built-in loop qualification tools”, by Martin Nilsson, Masters Degree Project Stockholm, 2005, report ID:IR-SB-EX-0509. Due to the fact that the degree of impulse noise protection today only is based on frequency or repetitiveness of the impulse noise, there is a risk that the selection of INP will not be optimal and that therefore performances losses will be unnecessarily high, i.e. it may easily be the case that the amount of INP applied is too high.